For determining the chemical and physical characteristics of stratospheric regions and especially for determining the effects of environmental conditions and the activities of mankind on the ozone belt above the earth, it is a common practice to transport, generally by balloon, air sampling devices to appropriate heights and to selectively open communication between the sampling device and the surrounding atmosphere to allow the samples to be collected.
It has been proposed, for the sampling of air at altitudes of 20 to 35 km, to utilize cryogenic samplers in which components of the sample are deposited by freezing or sublimation on a sampling finger or probe which is chilled by a liquid cryogen such as liquid nitrogen.
Samples of this type are effective for determining anthropogenic influences on the atmosphere, e.g. from Freons, fertilizers and the like, the samples being subjected to analysis in ESR spectrometers to determine the content of paramagnetic molecules such as those of NO.sub.2 and HO.sub.2.
Helium-filled balloons generally carry the sampling devices up to altitudes of 40 km and the sampling can take place over a period of several hours, whereupon the sampling unit is permitted to fall to earth by parachute and is recovered.
A sampling device for the purpose described is discussed in Ber. Bunsenges. Phys. Chem., 82 (1978) pp. 16-19 of D. Mihelcic et al. and utilizes a cryosampler with an evacuated or evacuatable vessel and a remotely controlled inlet nozzle opening when the device reaches the desired altitude. The nozzle is juxtaposed with a cooling finger held at a low temperature by contact with a liquid reservoir. The vacuum vessel is provided with a cryogenically cooled adsorber which maintains the reduced pressure in the vacuum vessel.
When the nozzle orifice is opened at the desired altitude, air is drawn into the vacuum vessel and initially flows along the cooling finger so that any carbon dioxide (which may be present in an amount of 320 parts per million) is frozen out and, in addition, natural and anthropogenic trace gases are collected. Most of the oxygen and nitrogen constituting the air is picked up by the adsorber to maintain the reduced pressure.
The unit, upon completion of its sampling mission, can land at a velocity of about 5 m/sec. and the landing site will vary in terrain from forest, to ravine, to pavement, to mountain or to river, depending upon the geography. Frequently access to the landed unit is difficult and the unit may have to wait 5 to 24 hours before it is recovered and the sample can be removed.
The unit described in this publication has been found to have numerous problems. For example, if the unit lands on its side or in an inverted manner, the liquid coolant is rapidly lost and the sample may likewise be lost. In addition, transfer of the sample to the spectrometer frequently brings the sample into contact with ambient air and reduces the precision of the results. The number of samples which can be taken with the unit is limited.